There is a need for an encoding system that is robust against frame loss in the encoding of voice data in voice communication that uses packets, such as VoIP (Voice over IP) or the like. This is because packets on a transmission path are sometimes lost in packet communication, of which Internet communication is a typical example.
One method for increasing robustness against frame loss is an approach to minimize the effects of frame loss by decoding one portion of transmission information when another portion of the transmission information is lost (see, for example, Patent Document 1). Patent Document 1 discloses a method whereby encoding information of a core layer and encoding information of an enhancement layer are packed into separate packets using scalable encoding for transmission. Applications of packet communication include multicast communication (one-to-many communication) using a network that includes a mixture of thick lines (broadband lines) and thin lines (lines having a low transmission rate). Scalable encoding is also effective when communication between multiple points is performed on the type of heterogeneous network described above, because it is not necessary to transmit different encoding information for each network when the encoding information is stratified according to each network.
The technique disclosed in Patent Document 2 is an example of a bandwidth-scalable encoding technique that has scalability (in the frequency axis direction) in the signal bandwidth and is based on a CELP (Code Excited Linear Prediction) system that is capable of high-efficiency encoding of voice signals. Patent Document 2 discloses an example of a CELP system for representing spectral envelope information of a voice signal using LSP (Line Spectrum Pair) parameters. A quantized LSP parameter (narrowband-encoded LSP) obtained by an encoding unit (core layer) used for narrowband voice is converted to an LSP parameter for wideband voice encoding using the equation (1) below,
                                                                        fw                ⁡                                  (                  i                  )                                            =                            ⁢                              0.5                ×                                  fn                  ⁡                                      (                    i                    )                                                                                                                        ⁢                              [                                                                            wherein                      ⁢                                                                                          ⁢                      i                                        =                    0                                    ,                  …                  ⁢                                                                          ,                                                            P                      n                                        -                    1                                                  ]                                                                                        =                            ⁢              0.0                                                                        ⁢                              [                                                                            wherein                      ⁢                                                                                          ⁢                      i                                        =                                          P                      n                                                        ,                  …                  ⁢                                                                          ,                                                            P                      w                                        -                    1                                                  ]                                                                        (        1        )            and the converted LSP parameter is used by an encoding unit (enhancement layer) for wideband voice, whereby a bandwidth-scalable LSP encoding method is created. In the equation, fw(i) is the i-th element of the LSP parameter in the wideband signal, fn(i) is the i-th element of the LSP parameter in the narrowband signal, Pn, is the LSP analysis order of the narrowband signal, and Pw is the LSP analysis order of the wideband signal. LSP is also referred to as LSF (Line Spectral Frequency).    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-241799    Patent Document 2: Japanese Patent Application Laid-Open No. 11-30997